This invention is directed to a breathable, stretch-thinned barrier film which uses a cavated filler for improved moisture vapor breathability at lower filler levels. The invention is also directed to breathable nonwoven fabric laminates containing the film.
Breathable, stretch-thinned films and nonwoven fabric laminates containing them are known in the art. The films are typically prepared by mixing a polyolefin, for example polyethylene or polypropylene, with a particulate inorganic filler, for example calcium carbonate. The mixture is cast or blown into a film. The film may have a single filler-containing layer, or may be coextruded with one or two outer skin layers for improved processing and later bonding to a substrate.
The films are rendered breathable to water vapor by stretching them uniaxially or biaxially, to 1.5-7.0 times their original dimension in one or both directions, at an elevated temperature which is below the melting point of the polyolefin. The stretching causes localized separation between the polyolefin and individual filler particles, resulting in void formation around the filler particles. The voids are bounded by thin polymer membranes, which may be continuous or broken between adjacent voids. The network of voids and thin polymer membranes creates a tortuous path through the film, through which water vapor may diffuse. However, the film remains substantially impermeable to liquid water.
The particulate inorganic filler, commonly calcium carbonate, is solid and much denser than the polyolefin in the film. Because the particles are solid, water vapor does not pass through them, but instead must pass around them within the voids. Thus, the filler particles themselves do not contribute to breathability, other than to facilitate the formation of voids. The relative impermeability of the particles, coupled with their high density, means that the loading of filler required to produce an effective breathable film is quite high on a weight percentage basis. When the filler is calcium carbonate, loadings of 45-70% by weight of the film, typically 50-60% by weight of the film, are common for films having desired levels of high water vapor breathability.
One drawback of high filler levels is that the correspondingly low level of polymer weakens the structural integrity of the films, rendering the films susceptible to puncture, rupture and tear. Only the polymer, and not the filler, contributes to the structural integrity of the films. Until now, the relative weakness of stretch-thinned breathable films has been overcome by laminating the films on one or both sides to a nonwoven web, such as a spunbond web. A polypropylene spunbond web, for instance, can serve as an effective load bearing constituent in a wide variety of personal care absorbent garments and medical apparel for which the breathable barrier laminates are employed. Another drawback of high filler levels has been build-up at the die lips during film extrusion.
There remains a need or desire for breathable, stretch-thinned films which provide high levels of breathability at lower filler (and correspondingly higher) polymer levels, and thus have improved strength.
The present invention is directed to a breathable, stretch-thinned film made from a mixture of a thermoplastic polymer and a cavated filler, and a nonwoven fabric laminate containing the film. The term xe2x80x9ccavated fillerxe2x80x9d includes filler particles which have hollow or cavity-containing molecular structures, i.e., molecular structures which are naturally hollow or contain cavities due to their chemistry. In one embodiment, stretch-thinned films of moderate to high breathability and high strength are formed from a blend of about 55-95% by weight thermoplastic polymer and about 5-45% by weight of a cavated filler. In another embodiment, stretch-thinned films of very high breathability and conventional strength are formed from a blend of about 20% to less than 55% by weight thermoplastic polymer and greater than 45% to about 70% by weight cavated filler particles. Particularly suitable cavated filler particles include cyclodextrins and molecules containing cyclodextrins.
Filler particles formed from cavity-containing molecules are believed to enhance the breathability of films because the particles themselves exhibit some moisture vapor transmission and, thus, contribute to the overall breathability of the film. Put another way, moisture vapor can pass through the filler particles as well as around the particles. Additionally, cavated filler particles generally have lower densities than solid filler particles formed of non-cavated inorganic structures. The increased water vapor transmission and lower density of the cavated filler particles allow the formation of breathable, stretch-thinned films having suitable water vapor transmission at lower filler loadings, based on weight percentage of the films. Alternatively, stretch-thinned films of exceptional breathability can be produced using higher loadings of cavated filler particles which are comparable to the loadings of conventional inorganic filler particles in conventional stretch-thinned films.
The breathable, stretch-thinned films of the invention may be used alone or in combination with one or more nonwoven webs in a wide variety of personal care absorbent garments and medical apparel. The increased film strength resulting from lower filler loadings may permit the stretch-thinned films to be used alone, without a separate load bearing constituent, in certain applications. Alternatively, the films may be combined with one or more nonwoven webs to provide breathable barrier laminates of improved structural integrity and/or breathability.
With the foregoing in mind, it is a feature and advantage of the invention to provide a breathable, stretch-thinned barrier film having improved strength and/or breathability, formed from a mixture of a thermoplastic polymer and a cavated particulate filler.
It is also a feature and advantage of the invention to provide a variety of breathable nonwoven fabric laminates which incorporate the breathable, stretch-thinned film made using a cavated filler.
It is also a feature and advantage of the invention to provide personal care absorbent garments and medical apparel which incorporate the films and laminates of the invention and exhibit improved durability and/or breathability.